1. Field of the Invention
The present invention relates to a digital watermark detecting device which detects from a digital content a digital watermark embedded therein and a method for detecting a digital watermark.
2. Description of the Related Art
Digital watermarking is the technique wherein to a digital content such as digitalized still image data, motion picture image data, audio data, music data and the like (digital copyright data), identification information including copyright owner of the contents and users, copyright information of copyright owner, utilization conditions of the contents, secrecy information at the usage thereof, copy control information and the like (hereinafter collectively referred to as a digital watermark) is embedded so that such watermark is not be easily perceived. Thereafter, a watermark is detected from the content at necessity, thereby copyright protection including usage control and copy control is conducted and secondary utilization of information is promoted.
For the purpose of prevention against illegal use, the digital watermarking technique must have such characteristics that a watermark should not be lost or modified by various operations and intentional attacks that may be normally made on the content (hereinafter referred to as robustness). In order to realize robustness against various operations and attacks, a digital watermark should be embedded with a certain strength of robustness so that the digital watermark can be detected even after the content is infected by such illegal operations or attacks.
As one of the methods for embedding and detecting a digital watermark, a method using a sequence of numbers. In the case of this method, in embedding digital watermark, a sequence of numbers “a” corresponding to a digital watermark to be embedded is generated, and the sequence “a” is superimposed on a content. In detecting the digital watermark, an objective content where the digital watermark has been embedded, and an original content before embedding the digital watermark are compared with each other, thereby a sequence of numbers “b” that corresponds to the sequence of numbers that may have been superimposed on the objective content is obtained, and thereafter correlation values between the obtained sequence of numbers “b” and sequences of numbers that may be embedded “w1” to “wn” are obtained. When the correlation value is a threshold or more, it is determined that the sequence of numbers “wi (i=1 to n)” is embedded in the objective content.
However, the method that requires an original content for detecting the digital watermark as mentioned above has a problem that the application range thereof is limited. For example, in a watermark detector which is located at a user side for the purpose of use control and copy control of a content, from its nature, it is impossible to use an original content. Provision of an original content without a digital watermark to the user side for use control and copy control makes embedding digital watermarking meaningless. Accordingly, for some of applications of digital watermarking, a method for specifying sequences of numbers superimposed on a content without using an original content is preferred. Nevertheless, in the prior art, there has not been an effective method for precisely detecting sequences of numbers embedded in a content without using an original content.